misc/ios: evaluate symlinks before comparing GOROOT and GOPATH

CL 163726 added workarounds to keep the iOS builders happy in
a symlinked temporary dir.

The workarounds also made the tests more realistic and improved
performance. Keep them but also handle symlinks better in the
exec wrapper.

Change-Id: Iaa2c03a1a3fb3aa5aaf62d79d52b63d5d8f11db5
Reviewed-on: https://go-review.googlesource.com/c/164698Reviewed-by: Bryan C. Mills <bcmills@google.com>

misc/ios/go_darwin_arm_exec.go

@@ -633,8 +633,12 @@ func subdir() (pkgpath string, underGoRoot bool, err error) {
 	if err != nil {
 		return "", false, err
 	}
-	if root := runtime.GOROOT(); strings.HasPrefix(cwd, root) {
-		subdir, err := filepath.Rel(root, cwd)
+	goroot, err := filepath.EvalSymlinks(runtime.GOROOT())
+	if err != nil {
+		return "", false, err
+	}
+	if strings.HasPrefix(cwd, goroot) {
+		subdir, err := filepath.Rel(goroot, cwd)
 		if err != nil {
 			return "", false, err
 		}
@@ -642,10 +646,14 @@ func subdir() (pkgpath string, underGoRoot bool, err error) {
 	}
 
 	for _, p := range filepath.SplitList(build.Default.GOPATH) {
-		if !strings.HasPrefix(cwd, p) {
+		pabs, err := filepath.EvalSymlinks(p)
+		if err != nil {
+			return "", false, err
+		}
+		if !strings.HasPrefix(cwd, pabs) {
 			continue
 		}
-		subdir, err := filepath.Rel(p, cwd)
+		subdir, err := filepath.Rel(pabs, cwd)
 		if err == nil {
 			return subdir, false, nil
 		}